This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Trophoblasts cells are derived from a common precursor cell. Differentiation occurs along one of several lines to become either invasive or noninvasive. The invasive cell, termed the extravillous trophoblast in the human or the trophoblast giant cell in the mouse, is one of the first cells to differentiate in the embryo and this differentiation occurs while the embryo is in a state of relative low oxygen tension. Several studies have demonstrated that this physiologic low oxygen is required for normal differentiation to occur. However, few studies have demonstrated the molecular mechanisms by which oxygen tension dictates the trophoblast cells lineage pathway. Those studies that have investigated this area have primarily focused on the role of hypoxia inducible factor 1a. While this transcription factor clearly plays a significant role in determining trophoblast cell fate, it is unrealistic to believe that it is the only means for a cell to respond to low oxygen tension. The overall hypothesis of this proposal is that oxygen tension, both dependant and independent of the hypoxia inducible factor complex, is critical in determining the pathway for cytotrophoblast cell differentiation during placental development. Because mouse placenta offers a scenario of diverse trophoblast differentiation, we will use the mouse model to study the role of oxygen tension in placentation. We intend to demonstrate that there is a critical level of HIF protein required to induce trophoblast giant cell formation and that this level can be achieved by low oxygen tension or pharmacologic manipulation. Then, we will decrease HIF protein levels with siRNA and demonstrate that trophoblast cells also respond to low oxygen tension with HIF-independent oxygen sensitive pathways. Finally, we will demonstrate a physiologic effect of oxygen tension on mouse placentation. These experiments will provide important insight into the mechanisms by which normal placentation occurs and help to understand the physiology of abnormalities of pregnancy including IUGR and preeclampsia. Equally as important, understanding the molecular mechanisms or oxygen responsiveness will help to provide insight into other pathologies including stress repair and tumorogensis. This project is novel because it will make distinctions between HIF dependant and independent oxygen response systems and begin to demonstrate the interaction of these systems.